1. Field of the Invention
The present invention relates to a temperature sensor incorporating a thermistor element that is used to detect temperature and, more particularly, to a temperature sensor required to have heat resistance up to a temperature of about 1000.degree. C. which is suitable for use in, for example, an exhaust gas temperature sensor that is installed on a catalytic converter, and so forth, of an automobile exhaust system and detects abnormal temperatures or catalyst deterioration and so forth.
2. Description of the Related Art
This type of temperature sensor typically provides a thermistor element, having a certain temperature-resistance characteristic, at the end of a wiring member for the acquisition of thermistor signals. This end section is covered with a bottomed cylindrical metal case resulting in a composition in which the thermistor element is housed in the space formed by the metal case and the wiring member end section.
Normally, by filling insulating powder into a space between a metal outer cylinder and core wires, the composition of the wiring member is such that the core wires are insulated and retained in the outer cylinder, and these core wires are connected to electrode wires provided on the thermistor element.
Since a thermistor material composing the thermistor element is comprised of an oxide semiconductor, the resistance characteristics (temperature-resistance characteristics) of the thermistor section fluctuate according to the oxygen partial pressure of the ambient atmosphere of the thermistor element.
In this type of temperature sensor, since the thermistor element is housed in a metal case, oxidation of a heat-resistant metal occurs at high temperatures (e.g., 700.degree. C. and above), and the oxygen partial pressure of the internal atmosphere decreases. In a harsh reducing atmosphere, a phenomenon occurs in which oxygen is temporarily taken from the oxide semiconductor of the thermistor section, resulting in a disturbance in the above resistance characteristics at high temperatures.
Therefore, it is necessary to provide the thermistor element housed in the metal case with suitable ventilation to stabilize the thermistor characteristics. Here, in the case of a temperature sensor employing a double tube structure in which the periphery of the wiring member is further covered with a metal cylinder, etc. (e.g., that described in FIG. 8 of Japanese Patent No. 2621488), a ventilation pathway can be formed by using the gap between the wiring member and the metal cylinder.
More recent sensors, however, are required to accurately detect temperature over ranges of low temperature variation and perform on-off control for assessment of catalyst deterioration and so forth, thus requiring an even faster response. Consequently, there is a need to make the outer diameter of the temperature-sensitive section smaller to increase the speed of the sensor response. The above-mentioned double tube structure is structurally complex and unsuited for accommodating this need.
On the other hand, in the case of a single tube structure in which the end section of a wiring member provided with a thermistor element is covered only with a bottomed, cylindrical metal case, although it is advantageous to make the outer diameter of the temperature-sensitive section smaller, the ventilation pathway for the thermistor element must then be formed inside the wiring member.
However, as was mentioned above, since the wiring member employs a composition in which the core wires are insulated and held in a metal outer cylinder by insulating powder filled into the outer cylinder, it cannot be expected to provide good ventilation. In addition, when the outer diameter of the temperature-sensitive section is made smaller to increase the response speed, since this requires that the wiring member has a smaller diameter, it becomes difficult to secure ventilation. Therefore, the need arises to clarify the quantity of ventilation required to stabilize the thermistor characteristics.
Therefore, in consideration of the above points, the object of the present invention is to provide a temperature sensor incorporating a thermistor element housed in a metal case and a wiring member for acquisition of signals from said thermistor element, wherein the wiring member employs a ventilation composition that provides stable resistance characteristics of the thermistor element, and to provide a manufacturing process for a temperature sensor incorporating such wiring member composition.